A gene is considered a candidate gene for type 2 diabetes in Pima Indians if 1) it has a known physiological function in a pathway relevant to type 2 diabetes/obesity or 2) it is associated with diabetes/obesity in another human population or in an animal model. Candidate genes analyzed in the past year include: PPARg2, PGC-1, IRS-1, IRS-2, FOXC2, and MCR4. Poymorphisms were identified in all of these genes and analyzed for association. As an example, the melanocortin 4 receptor (MC4R), has been identified as the cause of rare forms of monogenic obesity in both humans and rodents. Heterozygous coding mutations in MCR4 are implicated in 1 to 6% of early onset or severe adult obesity. As part of our efforts to identify obesity susceptibility loci in the Pima Indians, we screened MCR4 as a candidate gene. Sequencing of MCR4 in 96 severely obese Pima subjects identified two rare coding region variants and one common promoter variant. One coding variant predicts an arginine to lysine substitution at codon 165 (R165Q), while the second coding variant is a single base insertion (A) which predicts a premature STOP (TGA) at codon 37. The R165Q has been previously identified in other populations, and functional studies have shown that it dramatically reduces the activity of MCR4. In contrast, the single base insertion at nucleotide 100 has not yet been reported. Genotyping of the common promoter variant in a large cohort of Pima Indians indicates that this variant is highly associated with obesity in the general Pima population. We are currently pursuing functional studies on this gene.